Accept function as parameter in PHP

Question 1

I've been wondering whether is possible or not to pass a function as parameter in PHP; I want something like when you're programming in JS:

object.exampleMethod(function(){
    // some stuff to execute
});

What I want is to execute that function somewhere in exampleMethod. Is that possible in PHP?

Question 2

It's possible if you are using PHP 5.3.0 or higher.

See Anonymous Functions in the manual.

In your case, you would define exampleMethod like this:

function exampleMethod($anonFunc) {
    //execute anonymous function
    $anonFunc();
}

Question 3

function text($var) 

{

    if ( ! $var) {
        return;
    }
    do_something();

}

$var = text('');

echo gettype($var);
echo is_bool($var) ? "true" : "false";
echo is_string($var) ? "true" : "false";
echo is_null($var) ? "true" : "false";

returns:

NULL false false true

Question 4

Cracks knuckles

Technically the syntax is "correct" (it won't generate a fatal error) but the semantics of PHP render it effectively meaningless in its current form. Let's look at a few things first, namely how PHP handles the assignment of named functions to variables:

php > echo shell_exec("php -v");
PHP 5.4.16 (cli) (built: Oct 30 2018 19:30:51)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies

php > function speak($arg) {echo "{$arg}n";}
php > function give($arg) {return $arg;}
php > $speak = speak(4);
4
php > $give = give(4);
php > var_dump($speak);
NULL
php > var_dump($give);
int(4)

The function itself is executed upon assignment and its return value (NULL or otherwise) is assigned to the variable. Since we're only assigning the return value of a function's execution, trying to use this variable as a function name has no use:

php > $speak(4);
php > $give(4);
php >

Let's contrast this to the assignment of an anonymous function (a.k.a. a 'Closure'):

php > $min = 1; $max = 6;
php > $checkName = function ($value) use ($min, $max) {
php {   echo "value: {$value}n";
php {   echo "min: {$min}n";
php {   echo "max: {$max}n";
php { };
php > var_dump($checkName);
object(Closure)#1 (2) {
  ["static"]=>
  array(2) {
    ["min"]=>
    int(1)
    ["max"]=>
    int(6)
  }
  ["parameter"]=>
  array(1) {
    ["$value"]=>
    string(10) "<required>"
  }
}

Unlike some other languages, a closure is represented in PHP by an actual Object. Variables inside the 'use' clause are imported at the time the Closure was created; function parameters (i.e. $value) have their values captured when the Closure is called (hence why we see it noted as a required parameter and not a static value). The semantics of references within Closures aren't worth considering right now but if you want further reading, goat's answer to this question is a great start.

The major takeaway here is that the Closure's assignment to $checkName did not execute the Closure itself. Instead, $checkName becomes a sort of "alias" we can use to reference this function by name:

php > $checkName("hello ");
value: hello 
min: 1
max: 6
php >

Given how loose PHP is about the number of function parameters passed, a zero-parameter execution returns expected results:

php > $checkName();
value:
min: 1
max: 6
php >

Now let's take it another level deeper and define a function within a function:

php > function myOuterFunc($arg) {
php {   function myInnerFunc($arg){
php {     echo "{$arg}n";
php {   }
php { }
php > $myVal = myOuterFunc("Hello ");
php > var_dump($myVal);
NULL
php >

By now this result should make sense. Functions do not execute unless explicitly called; just because we call myOuterFunc doesn't mean we execute any function code defined inside of it. That's not to say that we couldn't:

php > function myOuterFunc($arg) {
php {   function myInnerFunc($arg){
php {     echo "{$arg}n";
php {   }
php {   myInnerFunc($arg);
php { }
php > $myVal = myOuterFunc("Hello ");
Hello 
php > var_dump($myVal);
NULL
php >

Which brings us back around to what is essentially your question: what about a named function inside of a Closure? Given what we've now discovered about function execution, we can generate a series of very predictable examples:

$min = 1; $max = 6;
$checkName = function ($value) use ($min, $max) {
  function question(){echo "How are youn";}
  echo "value: {$value}n";
  echo "min: {$min}n";
  echo "max: {$max}n";
};
php > $checkName("Hello ");
value: Hello 
min: 1
max: 6
php >

As expected, the named function's code inside the Closure is not executed because we have not explicitly called it.

php > $min = 1; $max = 6;
php > $checkName = function ($value) use ($min, $max) {
php {   function question(){echo "How are youn";}
php {   echo "value: {$value}n";
php {   echo "min: {$min}n";
php {   echo "max: {$max}n";
php {   question();
php { };
php > $checkName("Hello ");
value: Hello 
min: 1
max: 6
How are you
php >

Explicitly calling the inner function works just fine, provided we define that function before we call it:

php > $min = 1; $max = 6;
php > $checkName = function ($value) use ($min, $max) {
php {   echo "value: {$value}n";
php {   echo "min: {$min}n";
php {   echo "max: {$max}n";
php {   question();
php {   function question(){echo "How are youn";}
php { };
php > $checkName("Hello ");
value: Hello 
min: 1
max: 6
php >

php > $min = 1; $max = 6;
php > $checkName = function ($value) use ($min, $max) {
php {   echo "value: {$value}n";
php {   echo "min: {$min}n";
php {   echo "max: {$max}n";
php {   function question(){echo "How are youn";}
php {   question();
php { };
php > $checkName("Hello ");
value: Hello 
min: 1
max: 6
How are you
php >

So to the point of your questions then:

Yes it's legal and what you're attempting is possible but semantically meaningless in its current form.
Any named functions inside the Closure definitely do not reside in the global namespace, they are within the scope of their defining Closure. FWIW, the term "members" typically refers to class variables (usually called "properties" in PHP). While Closures are an Object and let you duplicate the functionality of instance variables found within classes, they should not be confused or construed with classes in any way.

3/4) Not how you're trying to use it, no. Nothing outside of the Closure has any concept of the functions inside; if in calling your Closure code, said code performs operations using the inner functions, then they will see the light of day. But there is no immediate way to reference those inner functions as if they were defined outside of the Closure's scope.

In other words, the only way you'll get those inner functions to execute is if a) that code is specifically executed by the Closure's code and b) you execute said Closure code.

Hope this helps.

Question 5

You can pass a property accessor to the method.

List<Class1> SortBy(List<Class1> toSort, Func<Class1, IComparable> getProp)
{
    if (toSort != null && toSort.Count > 0) {
        return toSort
            .OrderBy(x => getProp(x))
            .ToList();
    }
    return null;
}

You would call it like this:

var result = SortBy(toSort, x => x.maxSpeed);

But you could go one step further and write your own extension method.

public static class CollectionExtensions
{
    public static List<TSource> OrderByAsListOrNull<TSource, TKey>(
        this ICollection<TSource> collection, Func<TSource,TKey> keySelector)

        if (collection != null && collection.Count > 0) {
            return collection
                .OrderBy(x => keySelector(x))
                .ToList();
        }
        return null;
    }
}

Now you can sort like this

List<Class1> sorted = toSort.OrderByAsListOrNull(x => x.maxSpeed);

but also

Person[] people = ...;
List<Person> sortedPeople = people.OrderByAsListOrNull(p => p.LastName);

Note that I declared the first parameter as ICollection<T> because it must fulfill two conditions:

It must have a Count property
It must be an IEnumerable<T> in order to be able to apply the LINQ method OrderBy.

List<Class1> is an ICollection<T> but also an array Person[] as many other collections.

So far, I have shown how you can read a property. If the method needs to set a property, you need to pass it a setter delegate as well

void ReadAndWriteProperty(Func<Class1, T> getProp, Action<Class1, T> setProp)

Where T is the type of the property.

Question 6

If you don't need anything fancy like delaying the evaluation of the function name or its arguments, you don't need eval:

function x()      { echo "Hello world";          }
function around() { echo before; $1; echo after; }

around x

does what you want. You can even pass the function and its arguments this way:

function x()      { echo "x(): Passed $1 and $2";  }
function around() { echo before; "$@"; echo after; }

around x 1st 2nd

prints

before
x(): Passed 1st and 2nd
after

Accept function as parameter in PHP

Answers

php,function